The present applicant has previously proposed a bolt or nut tightening device which comprises a tightening torque measuring unit connected to the output shaft of a power tightening machine, and a socket having a reaction receiver and connected to the output shaft of the measuring unit (JP2006-21272A).
With proposed bolt or nut tightening device, the power tightening machine serving as input means can be an existing one, and the tightening torque measuring unit is adapted to indicate actual values of tightening torque.
The tightening torque measuring unit comprises a strain gauge provided on a tubular body connected to an outer output shaft of the power tightening machine coaxially therewith. The tubular body is externally provided with a circuit board for converting the amount of strain of the strain gauge to a corresponding amount of tightening torque, an indicator for indicating the amount of tightening torque and a battery. The circuit board, the indicator and battery are covered with a tubular casing coaxial with the tubular body.
To accommodate the circuit board, the indicator and the battery in an annular space inside the tubular casing around the tubular body, the casing needs to have a length and an inside diameter fully sufficient to position these components to be covered within the casing.
The tubular casing serves to protect the components of the tightening torque measuring unit and must have high rigidity for use in work environments such as work sites wherein the device is likely to collide with workpieces or to slip off from the hand to fall.
This gives the tightening torque measuring unit an increased outside diameter, a larger length and greater weight.
The tightening torque measuring unit itself has no reduction mechanism incorporated therein, so that large bolts or nuts to be tightened by controlling the tightening torque require an expensive power tightening machine of large size serving as input means and having a reduction mechanism for effecting a great reduction.
If a great tight torque measuring unit is connected to the power tightening machine of large size, the machine is not fully satisfactory in maneuverability and ease of use.
The torque indicator of the tightening torque measuring unit is adapted to indicate the digits of a numerical value as arranged axially of the tubular casing and therefore has the problem that it is difficult for the worker to read the values while operating the power tightening machine.
An object of the present invention is to provide a bolt or nut tightening device wherein use can be made of an existing inexpensive small power tightening machine or a manual wrench as input means and which can be prevented from becoming greater in outside diameter and length although including a planetary gear mechanism, the device being highly maneuverable and easy to use and having a tightening torque indicator which is easy to watch.